The Stirling cycle engine has been known generally for decades and relies upon the pressure variations of a mass of working fluid confined in a work space. These pressure variations are caused by the alternate heating and cooling of the working fluid which is forced by a displacer piston between communicating hot space and cold space portions of the work space. One engine which offers advantages is the free-piston Stirling engine of the type illustrated and described in the U.S. patents of W. T. Beale U.S. Pat. Nos. 3,552,120; 3,645,649; and 3,828,558 and French Pat. Nos. 1,407,682 and 1,534,734. These engines require no synchronizing mechanical connection between their displacer piston and power piston.
In the free-piston Stirling engine, the displacer and power piston reciprocate at the same frequency but with position-time characteristics which are different and not in phase.
In order to minimize collision between the two pistons and between the displacer and the end wall of its cylinder and to provide some force in opposition to gravitational forces, a helical spring has in the past been linked to the displacer and connected between the end of the displacer rod and its cylinder housing. Such mechanical springs, however, suffer from several disadvantages. They tend to wear out by flaking, fatiguing and ultimately failing. In fact, such springs have demonstrated a higher than usual failure rate when used in the Stirling engine environment.
Mechanical springs are also inadequate for use in Stirling engines because each particular spring has a single force constant as determined by its material, geometrical configuration and Hooke's law. Because a free-piston Stirling engine may be initially charged within a broad range of working gas pressures, different mechanical springs are required for effective operation under different gas charge pressures.
Still another problem with mechanical springs is that they apply radially directed or side forces to the displacer rod in addition to the primary axial forces for which they are included in the engine. These side forces increase the static friction between the walls of the displacer rod and its slideably engaged cylinder and therefore impede the initial starting of such an engine.
Those springs which are used in the prior art engines have such small spring force constants that they have no significant effect on the frequency of operation or the operating characteristics of the engine. The frequency of operation and phase relationships in such engines are effectively a function of the mass and geometry of the displacer, power piston and cylinder housing and the characteristics of the working fluid. Consequently, during the operation of the conventional engine, it has been the pressure variations in the working space which primarily controlled the movement of the displacer and power piston.
It has always been a problem to start or initiate the oscillations of a free-piston Stirling engine because the conventional engine has a tendency to couple energy from the displacer to the power piston during any start up motion. This loss of displacer energy damps displacer operation and would more advantageously be used during start up to increase the amplitude of the displacer oscillations.
Additionally, it has in the past been impractical to vary the frequency of operation or other operating characteristics of prior art engines during their operation because their physical dimensions can not be conveniently varied without introducing substantial complexities into the engine.
Furthermore, multi-ended free-piston Stirling engines have in the past generated substantial mechanical vibration.